Pressure-compensated transducer



June 1, 1965 D. c. BRATE PRESSURE-COMPENSATED TRANSDUCER Filed Jan. 29, 1963 IN VENTOR. Donald C, Brute f -m?% State This invention relates generally to electroacoustic trans ducers, and more particularly to an improved pressurecompensated transducer.

Heretofore various pressure-compensated electroacoustic transducers have been suggested including hydrophones that, in order to achieve the required high electrical capacitance and high sensitivity particularly at low frequencies incorporated a relatively large amount of piezoelectric ceramic as would be aiforded, for instance, by a ceramic cylinder. Although the performance of some of these prior designs has been acceptable, they have not been entirely satisfactory from the standpoint of ability to withstand severe shock impact forces to which they are subjected, as well as from the standpoint of cost of manufacture. Transducers designed as hydrophones which employ piezoelectric ceramic cylinders must have considerable additional protective supporting structure to protect them against damage from impact or shock. For example hydrophones employed in sonobuoys must Withstand a considerable impact which occurs as the air dropped sonobuoy strikes the water. Also, tactical use of such hydrophones in a sonar sensing array towed from a ship at sea must be rugged enough to withstand extremley rough handling such as occurs when the array is launched from the ship or is retrieved from the ocean to the deck of the ship.

Other prior art pressure-compensated electroacoustic transducers employing a diaphragm and delicate mechanical linkage members are also susceptible to damage from impact. Further, such designs of necessity incorporate a multiplicity of fragile parts and are therefore more difficult and hence more costly to assemble in production.

It is, therefore, an object of the present invention to provide an improved pressure-compensated transducer which minimizes the amout of piezoelectric material required, without sacrificing sensitivity or electrical capacity.

Another object of this invention is to provide an improved pressure-compensated transducer which has a high resistance to shock and impact and which is simple in construction and reliable in operation.

Another object of this invention is to provide an improved pressure-compensated transducer which affords high sensitivity to acoustic pressure waves and can be electrically connected so as to be substantially insensitive to acceleraiton reaction pressures due to motion of the transducer.

These and other objects and advantages will be apparent to those skilled in the art when the following description of the invention is read in conjunction with the appended drawings, and the novel feature will be particularly pointed out hereinafter in connection with the appended claims.

In the appended drawings which are illustrative of the principles of the invention:

FIG. 1 shows a longitudinal sectional view of a pressure-compensated hydrophone in the assembled condition;

FIG. 2 is a sectional view of the hydrophone as taken along lines I-I in FIG. 1;

FIG. 3 shows a sectional view of another embodiment of a pressure-compensated hydrophone;

FIG. 4 shows a sectional view of the embodiment of the hydrophone as taken along lines IIII in FIG. 3;

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FiG. 6 shows an enlarged sectional view of another form of bilaminated disc type piezoelectric element;

FIG. 7 shows a diagrammatic sectional view of the reaction of the pair of piezoelectric disc elements of FIGS. 1 and 3 when subjected to acceleration forces; and

FIG. 8 shows a diagrammatic sectional view of the reaction of the pair of piezoelectric disc elements of FIGS. 1 and 2 when subjected to acoustic pressure waves.

In the embodiment of the invention, illustrated in FIGS. 1 and 2, the transducer 11 which is shown comprises a pressure-compensated hydrophone body 12 carried within a boot or hose assembly 13. Assembly 13 comprises a hose section having a pair of end caps 16 and 17 being held in fluid-tight relationship therewith by means of circular clamps 18 and 19. End cap 16 is provided with a suitable threaded opening into which an electrical connector member 21 is held in fluid-tight relationship. Assembly 13 not only serves to house hydrophone body 12 but defines a fluid reservoir 22 filled with a compressible fluid such as silicone oil surrounding body 12. Hose section 14 is to be constructed to a flexible material such as neoprene and serves as a yieldable diaphragm that is responsive to ambient hydrostatic pressure as transducer 11 is lowered into or raised from the depths beneath the surface of the ocean. Hose 14, being of neoprene, is substantially transparent to acoustic pressure waves that are transmitted through the ocean to the oil in reservoir 22 and thence to hydrophone 12, whose construction and operation will be more fully described hereinafter.

Hydrophone body 12 includes a casing 23 which is generally cylindrical in shape and has a central opening. Casing '23.should be of a rigid material and aluminum has been found to be satisfactory. A flange portion 24 extends circumferentially about the outer periphery at the central part of casing 23. Casing 23 is held in position within assembly 13 by means of clamp member 26 which restrains hose 14 against flange 24. Flange 24 is relieved so as to provide openings 27 and 28 (FIG. 2) permitting free fluid flow within reservoir 22. Opposite ends of casing have annular grooces 31 and 32 formed therein for respectively receiving a pair of piezoelectric disc elements 33 and 34. Elements 33 and 34 are spaced axially on the longitudinal axis 36 of hydrophone body 12 and serves aspart of the end wall means for reservoir 37. Removable plug 38 is carried in a threaded opening in casing 23 through which reservoir 37 may be filled with oil.

Elements 33 and 34 comprise flat circular backing discs 41 and 42 that are connected into grooves 31 and 32 as by cementing. Discs 41 and 42 which are of a yieldable aluminum and the cement is of a non-conducting epoxy resin whereby they are effectively electrically insulated from casing 23. By means of a conducting epoxy resin pairs of piezoelectric bodies 43, 44, and 46, 47 are mounted on and electrically connected to opposite faces of discs 41 and 42. Bodies 43, 44, 46 and 47 are thin, flat, circular polarized ceramic material such as leadzirconate titanate. Conducting electrodes are formed on opposite surface areas of each body in the usual manner. From the foregoing description it will be apparent that elements 33 and 34 are of a trilaminate construction.

Reservoir 22 and reservoir 37 are both filled with a compliant oil such as silicone oil. In order to provide fluid-pressure communication and hence pressure equalization between reservoirs 22 and 37, a capillary tube 48 is cemented into an opening formed in disc 42. Actually the 48 serves as a means for providing restricted fluid-pressure communication between reservoirs 22 and 37. Pressure equalization is achieved in the following manner. As transducer 11 is submerged into the ocean an increased hydrostatic pressure acts upon the oil in the reservoir 22 causing oil to flow through tube 48 into reservoir 37 as the oil is compressed, thereby equalizing the pressure on both r 3 sides of piezoelectric elements 33 and 34. Conversely, when the hydrophone is subjected to a lower hydrostatic pressure from an equilibrium condition, oil will be transferred from reservoir 37 to reservoir 22 'as the oil expands and thus pressure equalization is achieved. 7

The manner in which transducer 11 senses acoustic pressure waves will now be described. Piezoelectric ele ments 33 and 34 are responsive principally to acoustic pressure waves above a predeterminable cut off frequency. That is to say that acoustic pressure fluctuations trans mitted through the sea and into the oil in reservoir 22, provided such fluctuations are above a predetermined frequency, will not be transmitted into reservoir 37. This so-called cut off frequency can be predetermined as it is dependent upon the resonant frequency of a resonant system comprising reservoirs 22 and 37, the oil, and tube 43. The manner in which the area of reservoirs 22 and 37, the viscosity and compressibility of the oil, and the diameter and length of tube 48 influence this resonant system, is known in the art as the HelmholtzPhenomenon. For example, if transducer 11 is to be used over the band from 50 c.p.s. to kc. and is to have essentially, constant response over this band of frequencies, the cut oif or resonant frequency can be established at below 50 c.p.s.

Referring now to FIGS. 1, 7, and 8, the electrical network interconnects the ceramic bodies 43, 44, 46, and 47 in series will now be described. Standoff 49, which is mounted on disc 41 by suitable means such as epoxy resin cement, supports main terminal 51 which is connected by means of lead 52 to one'surface of ceramic'body 43. The opposite surface of body 43 is connected through disc 41 to one surface of ceramic body 44, the latters opposite surface being connected to the near surface of ceramic body 46 by means of lead 53. The other surface of ceramic body 46 is connected through disc 42 to the surface of ceramic body 47., Theropposite surface of disc 47 is connected by means of lead 54 to insulated feed through 56, the latter being cemented as by an epoxy resin into an opening provided in disc 42. Feed through 56 is then connected by means of lead 57 to insulated feed through 58 which supports main terminal 59, feed through 56 being cemented by epoxy resin into an opening in disc 41.

To realize optimum performance, elements 33 and 34 should be electrically balanced. To accomplish this balancing an adjunct electrical network is necessary which includes an insulated feedthrough 61 which is cemented by epoxy resin into an opening provided in disc 41. Feed through 61 supports terminal 62 one end of the latter being connected by means of lead 63 to the surface of'ceramic body 44 towhich lead 53 is connected. The actual electrical balancing of the circuit may be affected quite simply by adding a suitable electrical reactance (not shown) between terminal 62 and either of main terminals 51 and 59.

To complete the electrical network main terminals 51 and 59 are connected respectively by leads 66 and 67 to connectorterminals 68 and 69 for transmission of signal voltage from elements 33 and 34 to an external load (not shown) through a connector cable, a portion of which is identified by Arabic numeral 71.

Referring specifically to FIG. 8 when transducer 11 is employed as a hydrophone in an operational environment beneath the surface of the ocean, acoustic pressure waves that are generated in the ocean by a remote target are illustrated diagrammatically by arrows identification by capital letter P. These pressure waves acting simultaneously upon the faces of elements 33 and 34 that are exposed to reservoir 22 (FIG. 1) excites elements 33 and 34 V in phase and their resulting generated voltage outputs are added and will appear across terminals 51 and 59.

Referring specifically to FIG. 7, should elements 33 and 34 be subject to a mechanical input such as caused by .an acceleration of transducer 11 axially as indicated by an arrow identified by capital letter A, elements 33 and 34 will deflect simultaneously in the same direction as shown due to the acceleration reaction forces. It is readily apparent that the mechanical input due to acceleration reaction forces excites elements 33 and 34 out of phase and their resulting electrical output voltages cancel and substantially no voltages due to such inputs will appear across terminals 51 and 59.

Referring to FIGS. 3 and 4 another illustration of the invention shows a transducer 75 comprising a boot or hose assembly 13 which has already been described in connection with FIG. 1 and a hydrophone body 76 carried within .the assembly 13. Body 76 includes a casing 78 held in position by means of clamp 26 which restrains hose section 14 against radially outwardly extending flange 77 formed centrally on casing 78. As before, assembly 13 defines a reservoir 22' surrounding casing 78. Flange 77 has two openings'79 and 80 (FIGURE 4) which permit free fluid flow within reservoir 22.

Casing 78 of hydrophone 76 is of generally cylindrical shape having a central opening which serves as a reservoir 81 and is to be filled with a suitable complaint fluid such as silicone oil and is otherwise similar to casing 23 (FIG. 1) but with radially inwardly extending flange portions 82 and 83 at opposite ends thereof. Flange portions 32 and 33 together with disc type piezoelectric elements 33 and 34 form the end wall means for reservoir 81. Elements 33 and 34 which have already been described previously in connection with FIGS. 1, 7 and 8 are axially spaced along the longitudinal axis line identified by numeral 36 of hydrophone body 76 and are cemented as by anon-conducting epoxy resin respectively into annular grooves 36 and 87 in flange portions 82 and 83.

One end of casing 78 is enclosed by a diaphragm 88 which is held thereon by means of a clamp 89. Diaphragm 88 is of a substantially acoustic transparent material such as molded neoprene and forms a reservoir 91' enveloping the face of element 34 remote from reservoir 81. Reservoir 91 is to be filled with a compressible oilof the same type as carried in reservoir 81. Diaphragm 88 is of course flexible and thus responsive to hydrostatic pressure in reservoir 22.

Capillary tube 48 is cemented into an opening provided in flange portion 83 rather than in disc 42 (as shown in FIG. 1) and provides pressure equalization between reservoirs 81 and 91 as previously described for reservoirs 22 and 37 (FIG. 1). Elements 33 and 34 of hydrophone body 76 are electrically interconnected exactly as herein before described, except that insulated feed throughs 56, 58, 61 respectively'are carried in suitable openings pro vided in flange portions 32 and 83 and standoff 49 also is mounted on flange 32. A distinct difference between transducer 11 and transducer is the latter has three rather than two reservoirs. This feature provides the advantage that reservoir 22 can be filled with a much less costly fluid than silicone oil. desirable to fill it with castor oil. It will also be apparent that hydrophone body 76 canbe easily adapted to operate as a pressure-compensated transducer independently of boot assembly 13.

Referring to FIG. 5 a bilaminated type of piezoelectric disc element 93 is carried in grooves 87 of flange 83 of casing 73 (FIG. 3.) Element 93 comprises a pair of ceramic bodies 96 and '97 having electrodes formed on their opposite surface areas and being bonded together with the electrodes of their joined surface being electrically connected. This connection can be accomplished by using a conducting epoxy resin as the bonding cement. Capillary tube 48, rather than carried in flange83, is cemented as'by an insulating epoxy resin in an opening provided in the geometric center of element 93.

Referring to FIG. 6 still another form of bilaminated ceramic disc element 99 is illustrated wherein the yieldable backing disc 42 is mountedin groove 87 in flange 83 of casing 78 (as shown in FIG..3). Only one thin ceramic body 47, however, is mounted upon disc 42. Capillary tube 48 is carried in flange 83 as previously described.

For instance, it may be 7 It is to be understood that elements 93 and 99 as described in FIGS. 5 and 6 can be used in pairs and be either electrically connected in series or in parallel depending upon the use to which the transducer mi ht be put.

From the foregoing it will be apparent that a new and useful transducer invention has been described which incorporates a number of significant features. In the hydrophone construction as disclosed herein the axially spaced piezoelectric sensing elements are rugged enough to withstand a considerable shock should the hydrophone be subjected to impact. It will also be readily seen that the sensing elements 33 and 34 can easily be protected from blows that might otherwise impinge directly thereon without impairing sensitivity.

The feature of acceleration cancellation is of considerable importance. Should either of transducers 11 or 75 be employed in an environment in which it will be sub jected to severe mechanical inputs such as when moored by a cable that is attached to a buoy floating on the waves of the oceans, or when employed as a sonar device and attached at the end of a cable and towed beneath the surface of the sea as by a helicopter. As is readily apparent, the illustrated hydrophone design requires the use of a minimum amount of piezoelectric ceramic material, thereby making possible a considerable cost saving without sacrificing sensitivity. The hydrophone body in 12 or 76 are themselves extremely rugged. The end wall means for reservoir 37 (FIG. 1) and 81 (FIG. 3) which include elements 33 and 34 and the flange members 82 and 83 in the case of the modification shown in FIGS. 3 and 4 represent a simple and compact construction that can be easily assembled.

Although in the foregoing disclosure of the invention, particular reference has been made to an electroacoustic transducer employed as a hydrophone, it should be understood that the transducer is equally well suited to be used as an acoustic projector.

It is to be understood further that various changes in the details, materials, and arrangement of parts, which of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. A pressure-compensating transducer comprising: a rigid casing having a central cavity to define a first fluid reservoir; end wall means for said reservoir including a pair of axially spaced piezoelectric disc elements; means for electrically interconnecting said pair of disc elements to provide cancellation of generated voltages resulting from acceleration reaction forces acting on said transducer; means including a flexible diaphragm for defining a second fluid reservoir in fluid-pressure transmitting relationship with at least one of said disc elements; pressure equalization means for providing restricted fluidpressure communication between said first and second reservoirs.

2. The combination set forth in claim 1 wherein each of said end wall means include a yieldable backing disc, means for connecting said backing disc in fluid-tight relationship with said casing, and a piezoelectric body bonded on at least one face of said backing disc; and wherein said pressure transfer means includes a tube having a restricted opening therein, and means for connecting said tube in fluid-tight relationship in an opening provided in one of said end wall means said fluid reservoirs being filled with a single compliant fluid.

References Cited by the Examiner UNITED STATES PATENTS 2,126,438 8/38 Williams 310--8.6 2,448,365 8/48 Gillespie 34010 2,923,916 2/60 Woodworth 34010 3,002,179 9/61 Kuester 34010 3,018,466 1/62 Harris 340-10 X 3,048,815 8/ 62 Thurston et al. 34010 CHESTER L. JUSTUS, Primary Examiner.

LOUIS H. MYERS, Examiner, 

1. A PRESSURE-COMPENSATING TRANSDUCER COMPRISING: A RIGID CASING HAVING A CENTRAL CAVITY TO DEFINE A FIRST FLUID RESERVOIR; END WALL MEANS FOR SAID RESERVOIR INCLUDING A PAIR OF AXIALLY SPACED PIEZVELECTRIC DISC ELEMENTS; MEANS FOR ELECTRICALLY INTERCONNECTING SAID PAIR OF DISC ELEMENTS TO PROVIDE CANCELLATION OF GENERATED VOLTAGES RESULTING FROM ACCELERATION REACTION FORCES ACTING ON SAID TRANSDUCER; MEANS INCLUDING A FLEXIBLE DIAPHRAGM FOR DEFINING A SECOND FLUID RESERVOIR IN FLUID-PRESSURE TRANSMITTING RELATIONSHIP WITH AT LEAST ONE OF SAID DISC ELEMENTS; PRESSURE EQUALIZATION MEANS FOR PROVIDING RESTRICTED FLUIDPRESSURE COMMUNICATION BETWEEN SAID FIRST AND SECOND RESERVOIRS. 